1. Field of the Invention
This invention relates to a constant voltage supplying circuit which is interposed between a voltage generation circuit and a load, stabilizes an output voltage of the voltage generation circuit to a desired voltage value and supplies the voltage to the load.
2. Description of the Related Art
FIG. 1 shows the constitution of a constant voltage supplying circuit as an example of the prior art.
In the drawing, reference numeral 1 denotes a voltage generation circuit which generates a positive voltage VDD1 at the time of non-load; 1A denotes a voltage output terminal of the voltage generation circuit 1; 2 denotes an internal resistance of the voltage generation circuit; and VD1 denotes a voltage of the voltage output terminal 1A when the load is connected. Reference numeral 3 denotes a constant voltage supplying circuit and reference numeral 4 denotes a Zener diode which constitutes the constant voltage supplying circuit 3. The Zener voltage (hereinafter called "VZ") of this Zener diode is set to be lower than the voltage VDD1 output by the voltage generation circuit at the time of non-load. Symbol VOUT denotes the output voltage of the constant voltage supplying circuit, GND denotes an earth voltage and reference numeral 5 denotes a resistor as the load.
In the constitution described above, the constant voltage supplying circuit 3 stabilizes the voltage VD1 output to the voltage output terminal 1A of the voltage generation circuit 1 so that it becomes equal to the Zener voltage VZ of the Zener diode 4 (VD1&gt;VZ&gt;0), and supplies it as the output voltage VOUT to the load resistor 5.
However, the constant voltage supplying circuit 3 according to the prior art described above involves the problem that the output voltage VOUT to be supplied to the load fluctuates due to variance of characteristics of the Zener diode 4.
Originally, the Zener diode 4 itself is a device for obtaining a constant voltage by causing a current to flow therethrough. Therefore, another problem exists in that power consumption other than that of the load is great.
Further, when the Zener diode 4 is used as a component to be externally mounted, the number of components increases. When the Zener diode is fabricated inside an integrated circuit to avoid this problem, the number of production steps increases and eventually, the cost of production increases.